1. Field of Invention
The invention relates to an inverter, and particularly to an interleaving control type inverter.
2. Related Art
Referring to FIGS. 1 and 2, a conventional three-phase inverter aims to transform DC power Vdc to three-phase AC power to drive a load 100. The three-phase AC inverter consists of a plurality of transistors coupled in parallel. Transistors 111 and 112 are coupled in parallel, transistors 113 and 114 are coupled in parallel, transistors 115 and 116 are coupled in parallel, transistors 121 and 122 are coupled in parallel, transistors 123 and 124 are coupled in parallel, and transistors 125 and 126 are coupled in parallel. These transistors are generally insulated gate bipolar transistors (IGBTs). The gates of the transistors 111˜116 are controlled by first control signals PWM_R1, PWM_S1 and PWM_T1, namely, the upper arm control signals corresponding respectively to R, S and T phases. The gates of the transistors 121–126 are controlled by second control signals PWM_R2, PWM_S2 and PWM_T2, namely, the lower arm control signals corresponding respectively to R, S and T phases. Taking the R phase for an example, the transistors 111, 112, 121 and 122 are driven respectively by gate drivers 131˜134.
Parallel coupling is accomplished and controlled by coupling two sets of IGBTs of the same model number. Only one gate control signal is required to drive two transistors that are coupled in parallel at the same time.
The inverter depicted in FIGS. 1 and 2 still has some technical problems, such as current distribution, malfunctioning, low efficiency and capacity. More details are elaborated as follows:
Because the static and dynamic characteristics of IGBTs are not always the same, controlling with direct parallel operation results in different current flowing through two IGBTs while turned on in a static condition or switching dynamically. As a result, current distribution in the IGBTs is not equal. In serious conditions, the IGBTs could overheat and burn out.
As the transistors 111 and 112 use the same set of control signal to pass through gate control circuits 131 and 132 and drive the IGBTs (referring to FIG. 2), if one IGBT is opened or the actuation circuit is abnormal (such as signal interruption), in terms of the parallel structure, as long as one IGBT is turned on normally (i.e., the transistor 111 is normal), the overall output actuation is not affected. The actual load current waveform is also the same as the normal signal. Hence malfunctioning of the IGBT cannot be detected, and protection of the IGBT is difficult. Isolation of the malfunction is also not easy. Moreover, when one IGBT malfunctions, excessive current could flow through another IGBT. When the malfunction is not detectable, the other IGBT could burn out. Reliability is thus lacking.
The power loss of the general inverter can be classified as conduction loss and switching loss (including turn-on losses and turn-off loss). In general, a higher switching frequency of the IGBT has a more desirable output waveform, but the power loss is also greater, and the overall efficiency is lower. For an inverter of a greater capacity, to maintain a high switching frequency to achieve a desired waveform output is difficult.
In term of capacity, the safety current of the IGBT must be reduced as the switching frequency increases. Moreover, the dividing current is not equal when the IGBTs are coupled in parallel. Hence the total safety current has to be reduced.